1. Technical Field
The present invention is related to terminators for electrical connections in digital electronic applications. Specifically, the present invention is directed toward a terminator that allows for different modes of operation in order to balance performance and power consumption.
2. Description of Related Art
All electrical circuits exhibit non-ideal characteristics. The task of a circuit designer is to find suitable models for the behavior of the circuit being designed, so that the model closely approximates the actual behavior of a real circuit. Such a model may be used to obtain initial values for component parameters (such as resistances and capacitances). The designer may then further adjust or “tweak” the design as necessary to account for the inherent inaccuracies in the design model. This design approach is pervasive in Electrical Engineering.
Perhaps the most widely employed design assumption used in modeling electrical circuits is to assume that all signals in a circuit propagate through the circuit at an infinite speed (i.e., without any propagation delay). Although modern physics tells us that this assumption is entirely false, it is nonetheless a valuable analytical simplification and one that can be applied in an enormous number of settings. This assumption of a “zero propagation delay” breaks down, however, in very high speed or timing-sensitive circuits or when a signal must travel a significant distance before reaching its destination. When these sort of conditions occur, it then becomes necessary to adopt a different model.
When “zero propagation delay” can no longer be assumed, engineers typically employ what is known as a “transmission line” model, so called because propagation delay becomes a significant factor in the transmission lines used for power or telephone signal transmission, where electrical signals must travel relatively long distances, such that propagation delays become relevant. A transmission line has a characteristic impedance, which reflects the transmission line's tendency to impede the propagation of a signal travelling along the transmission line.
When a transmission line is terminated by a load (such as a resistor, transistor, or other circuit element), the impedance of the load and the characteristic impedance of the transmission line have a significant effect on the ability of the transmission line to accurately transmit the signal that is used to drive the load. The well-known “maximum power transfer theorem” from elementary circuit theory states that maximum power is transferred to the load when the load impedance matches the impedance of the driving circuit, and that less than the maximum amount of available power is transferred when there is an impedance mismatch. In the case of a transmission line, the available power that fails to be transferred to the load is “reflected” away from the load and back toward the driving circuit (note that this reflection phenomenon becomes perceptible in a transmission line model, since a non-zero propagation delay is assumed). The fraction of power that is reflected away from a mismatched load is given as                               γ          =                                                    Z                L                            -                              Z                0                                                                    Z                L                            +                              Z                0                                                    ,                            (        1        )            where ZL and Z0 are the load impedance and the characteristic impedance of the transmission line, respectively.
Where ZL and Z0 are matched, it is easy to see that γ=0. Thus, it is standard engineering practice to terminate transmission lines (or electrical connections modeled as transmission lines) in an impedance that matches the characteristic impedance of the line. For example, North American cable television cables are designed to be terminated with a 75 Ω load. Terminating a transmission line in a matching impedance not only results in an efficient transfer of power to the load, but also preserves signal integrity, as reflection due to impedance mismatching can cause signal degradation, including overshoot and undershoot (amplitude-related distortion), and jitter (phase-related distortion).
In modern high-speed digital circuits, transmission line effects can be observed in circuits of relatively small size. This is a particularly troublesome phenonmenon in board-level design, where the connections between integrated circuits (ICs) on a circuit board may act like transmission lines. In such instances, it is important to terminate the connections (pins) to integrated circuits in matching impedances, so as to reduce signal degradation due to transmission-line effects. A typical terminator circuit, as employed in the art, is depicted in FIG. 1.
Here resistor R1 and resistor R2 make up the terminator. Resistors R1 and R2 are connected to each other and transmission line 100 at node 102. Resistors R1 and R2 are also tied to ground and a positive voltage supply, respectively. Node 102 is the point of connection to an integrated circuit from transmission line 100. The impedance of the terminator (i.e., the two resistors together) is given by                                           R            total                    =                                                    R                1                            ⁢                              R                2                                                                    R                1                            +                              R                2                                                    ,                            (        2        )            as is well-known in the art. Resistors R1 and R2 prevent degradation of the input signal represented by voltage source Vsrc, by matching the characteristic impedance of transmission line 100, which connects voltage source Vsrc with the terminator comprising resistors R1 and R2.
One of ordinary skill in the art, however, will recognize that because resistors R1 and R2 themselves form a complete circuit with ground and the positive voltage supply, resistors R1 and R2 constantly dissipate power in the form of heat. In a very large scale integration (VLSI) circuit having many pins, even a small amount of current flowing through these terminator resistors can add up to an unacceptably high amount of power dissipation. Thus, there is a need for a terminator circuit that minimizes power dissipation in a very large scale integrated circuit design.